It is common practice to mix particulate solids, liquids and gases with motionless mixers having, as the name implies, no moving parts. Mixers of this category consist of baffles of various types arranged sequentially in a tube or pipe. By a process of division and recombination, separate input components can be mixed or dispersed within one another at the output of said tube or pipe.
Difficulties are often experienced, however, when mixing materials of widely disparate viscosities and/or very different flow rates. For example, in the polymer field, it is at times desirable to mix very small quantities of a low viscosity material within a much larger quantity of a high viscosity material. When this is done, the low viscosity material tends to tunnel through the mixing elements without blending with the high viscosity material to any great extent. As an example, one might wish to mix a stream flowing at a rate of 7 gpm of a polymer having a viscosity of 30 million centipoises with a second stream traveling at 0.035 gpm of 6 centipoise material.
A variety of approaches have been attempted to produce an initial degree of dispersion or mixing at the injection point of the low viscosity material. These approaches have included, by way of illustration, the use of a multiplicity of injection ports around the circumference of a pipe. A second approach has consisted of the use of a relatively small diameter pipe for carrying the low viscosity material which passes through the diameter of the main pipe carrying the high viscosity material. The small diameter pipe is configured to have a plurality of holes used for injecting the low viscosity fluid. A common problem of such devices having parallel path outlets is that the low viscosity fluid injection apertures become differentially plugged resulting in asymmetric distribution.
It is well known that one of the mechanisms that allows for mixing of fluids is diffusion. However, when dealing with high viscosity materials which typically produce laminar flow, diffusion rates are very small. It is known that the rate of mass transfer N of the diffusing component measured in moles per second per unit area is equal to the diffusivity D multiplied by the local concentration gradient ##EQU1## Thus, ##EQU2## Since D is small in high viscosity material, it is necessary to make the concentration gradient ##EQU3## large in order to maximize the value of the mass transfer rate N.
It is thus an object of the present invention to provide a motionless mixing device without the drawbacks of corresponding devices of the prior art.
It is yet another object of the present invention to present a motionless mixing device particularly useful in the mixing of two or more fluids having widely disparate viscosities.
It is yet another object of the present invention to present a motionless mixing device which maximizes the rate of mass transfer N to improve diffusion between the fluids to be mixed.
Referring again to the equation presented above, the rate of mass transfer N can be increased by decreasing dr. In principle, this can be accomplished by placing a relatively small diameter pipe across the diameter of a larger pipe or tube, the small diameter pipe having a thin slot along its length. The fluid component exiting the slot would be introduced in the form of very thin sheets, but the clogging problems discussed above would nevertheless plague this approach.
These problems were addressed in applicant's U.S. Pat. No. 4,808,007 filed on Aug. 27, 1987, the subject matter of which is shown in FIG. 1 appended hereto. As noted, the mixing device comprises a hollow tubular member 1 which is constricted at 9, said constriction comprising, for example, two orifices 5, 6 for passage of a relatively high viscosity fluid. As such, it is noted that applicant has taught an approved mixing device whereby at least two orifices which are preferably substantially cylindrical and whose axes are substantially parallel are shown as carrying a first fluid whereby a fluid entry port discharging a second fluid substantially between the two orifices at or near their points of tangency represents a mixing device superior to those which preceded it at least for the introduction of a low viscosity fluid into a mass flow of high viscosity materials.
Again, referring to FIG. 1, low viscosity fluid entry port 15 is shown to comprise an orifice located in hollow tube 20 which is shown radially extending through the sidewalls of an elongated hollow tubular member 1. The low viscosity fluid is caused to enter the motionless mixer through the hollow tube and its rate of discharge is controllable by pumping means (not shown).
As applicant has taught, hollow tube 20 passes radially through tubular member 1 through the center points of each orifice 5 and 6. Without orifices 5 and 6, low viscosity fluid entering a high viscosity fluid stream through entry port 15 would simply form a thin line stream as the fluids pass through hollow tubular member 1. By practicing the invention disclosed in applicant's U.S. Pat. No. 4,808,007, it was surprisingly determined that the low viscosity fluid 20 forms an elongated flat plane across the diameter of the pipe which greatly enhanced molecular diffusion between the low viscosity and high viscosity fluids. This increased the surface area available for diffusion by a factor typically 25 to 50 times while at the same time increasing the value of ##EQU4##
Apparatus, such as that shown in FIG. 1, has been successfully used to introduce and mix a relatively small amount of an additive into a viscous main product such as a thermoplastic polymer melt. Such melts have viscosities typically in a range of 50,000 to 10,000,000 centipoise. Additives can be colorants, lubricants, tackifiers and catalysts and, often, have viscosities much lower than the main product, for example, in the range of 1 to 1,000 centipoise. Low viscosity additives are commonly introduced at a rate of approximately 0.1% to 1% of the rate of the main product flow. Mixers such as those shown in FIG. 1 are generally used when it is necessary to accomplish the mixing task in a continuous or in-line fashion using static or motionless mixing devices since these are generally less expensive to install and maintain than mechanically driven mixing equipment. However, when additive viscosity and flow rate is small compared to the main flow, the number of static mixing elements must be increased to achieve an acceptable quantity of mixing. Although the device shown in FIG. 1 has adequately performed in the field, it has now been recognized that a more efficient means of mixing would be advantageous. Specifically, when mixing components which quickly react to one another upon contact, it has long been thought to be desirable to construct a motionless mixing apparatus which is capable of premixing components prior to their physical contact so that some degree of mixing is achieved before any reaction takes place.
These and further objects will be more readily appreciated when considering the following disclosure and appended claims.